Add support for `default_host` in `OperationsRestAsyncTransport`

Support for `default_host` is not yet supported in `OperationsRestAsyncTransport` which was added in https://github.com/googleapis/python-api-core/pull/700.

The reason that it was not included in googleapis/python-api-core#700 is because it is not yet supported in `google-auth`. `default_host` exists [here](https://github.com/googleapis/google-auth-library-python/blob/63f657174c603c077af3bbf7580b75b5f3b4943e/google/auth/transport/requests.py#L378-L380) in [class AuthorizedSession](https://github.com/googleapis/google-auth-library-python/blob/main/google/auth/transport/requests.py#L296) but there is no `default_host` in the async version [here in AsyncAuthorizedSession](https://github.com/googleapis/google-auth-library-python/blob/main/google/auth/aio/transport/sessions.py#L78)